In meteorology, it is known to measure the speed of air by means of a radar using the Doppler effect: the radar transmits trains of sinusoidal waves or pulses that are returned to the radar by particles in suspension in the air; if the air is moving in such a manner as to cause the particles to move away from or towards the radar, then the pulses returned to the radar present a phase shift relative to the pulses it transmitted, which shift can be used for calculating the radial speed of the particles relative to the radar, and thus the speed of the air carrying the particles. Speed can thus be determined without ambiguity providing the real speed of the particles lies within a so-called Nyquist range that depends on the pulse repetition rate (also known as repetition frequency). If the real speed of the particles lies outside that range, then the speed that is calculated is equal to the real speed modulo the width of the Nyquist range. The calculated speed is then said to be folded into the Nyquist range.
To increase the width of this range, it is known to increase the repetition rate of the pulses. Nevertheless, that leads to a certain number of drawbacks, and in particular to strongly stressing the transmitter, to the transmitter consuming a large amount of energy, and to a reduction in the range of the radar.
It is also possible to use a radar of longer wavelength. Nevertheless, such a radar is expensive.
It is also known to transmit bursts of pulses at first and second pulse repetition rates, with one rate taking the place of the other after each burst (the so-called dual pulse repetition frequency (PRF) method). By combining the speeds calculated from the pulses received in return from the pulses transmitted during successive bursts, the speed of the particles can be determined without ambiguity in a larger Nyquist range. Nevertheless, since the radar antenna is revolving continuously, the zone of the atmosphere to which a burst is transmitted at the first repetition rate is slightly different from the zone to which the following burst is transmitted at the second repetition rate. This results in inaccuracy in determining speed, and this inaccuracy increases when the radar is located in a zone where air speeds present high levels of local variation and where the radar rotates at a high speed.